1. Field of the Invention
The present invention relates to a process for separating a gas mixture in a separation unit of the type in which the gas mixture comes from a reaction unit and comprises, as main constituents, hydrogen (H2) and/or carbon monoxide (CO).
1. Related Art
Many industries use these gas mixtures which, in addition to CO and H2, in general also contain other constituents, especially carbon dioxide (CO2), methane and water, and also nitrogen and other impurities. Such mixtures constitute synthesis gases, called hereafter syngases, serving for various production processes. They are in fact used as raw material for the production of fluids, for example hydrogen, carbon monoxide, carbon dioxide or H2/CO mixtures in a predetermined ratio for the purpose of various chemical syntheses, especially the synthesis of methanol or of acetic acid, or the synthesis of reducing atmospheres for surface treatments, for metallurgical operations, etc.
These gas mixtures are generally obtained from one or more reaction media, which are themselves fed with gaseous feed fluids. As examples of reaction units treating such reaction media, mention may be made of units for the steam reforming of liquid or gaseous hydrocarbons, for autothermal reforming, for carbon dioxide reforming, for methanol reforming or cracking, or for partial oxidation of gaseous, liquid or solid compounds containing carbon and hydrogen.
Various separation techniques are used to prepare, from a gas mixture of this type feeding a separation step, at a feed pressure P0, one or more required fluid products. What is then obtained after the separation steps is at least two separate streams, at least two separate pressures. One of these streams is in general obtained at what is called a low pressure corresponding to a pressure that may be around P0/8 to P0/40, in such a way that this stream will be frequently compressed in order to be used, especially to utilize all or some of the molecules that it contains; the other stream is obtained at what is called a high pressure, corresponding to the feed pressure reduced by the pressure drops associated with the separation process; however, it is sometimes necessary for it to be compressed in order to use it.
Thus, when separation of a gas mixture of the above type is carried out by pressure swing adsorption (PSA), what is obtained is at least one relatively pure gas called the “PSA product” at a high pressure and a gas or gas mixture called the “PSA offgas” at low pressure. When separation is carried out by permeation through a membrane, some of the molecules pass through the membrane, forming a low-pressure gas stream called “permeate”, whereas other molecules, retained by the membrane, constitute the “residue”, which is a gas stream available at high pressure. Whatever the mode of separation, each of the two streams I and II contains all the compounds contained in the mixture to be separated, but in very different proportions (including sometimes traces).
When the desire is to use, as product, molecules contained in a “PSA offgas”, a “membrane permeate” or another gaseous fluid output at low pressure from a separation step, whether this be hydrogen, carbon monoxide or carbon dioxide for example, it is sometimes necessary:                to compress the gas in question in a compressor;        to separate, cryogenically or by scrubbing, the constituents present, so as to purify at least one type of molecule contained in the fluid in question; and        to associate compression with a cryogenic separation or a scrubbing.        
However, the possible combined presence of moisture and carbon dioxide may lead to the well-known phenomenon of carbon acid formation and consequently it may corrode the equipment used.
One known solution to this problem consists in providing equipment made of stainless steel, especially for steps of compressing said low-pressure fluid so that the equipment is protected from acid attack, or providing a step of predrying the gas stream when it has to be separated cryogenically, which is an expensive step as it is applied to a low-pressure stream.